Digital control of press synchronization

ABSTRACT

For automatically synchronizing the operation of a line of independently driven presses, each equipped with a variable speed drive, a digital control system in which respective digital signals representative of the instantaneous phase positions of the several presses in the line are each differentially compared against a digital reference signal representative of reference phase position. As a result of the differential comparisons, if the phase of any given press leads or lags the reference phase, a phase error is detected. This phase error is converted to an analog control signal which has a sense dependent on whether the press phase is leading or lagging the reference phase and a magnitude dependent on the amount of such lead or lag, and the control signal is fed back to the press drive to speed up or slow down the press as necessary to bring it back into step. The digital reference phase position signal cycles at the desired line operating speed, so that all presses operate in synchronism at that speed.

United States Patent [151 3,675,098

Heiberger July 4, 1972 [54] DIGITAL CONTROL OF PRESS Primary ExaminerBemard A. Gilheany SYNCHRONIZATION Assistant Examiner-Thomas Langer [72] Inventor: Francis E. Heiberger, Elmhurst, Ill. Atwmey wo]fe r voit & osann [73] Assignee: Danly Machine Corporation [57] ABSTRACT [22] Filed: June 11, 1970 For automatically synchronizing the operation of a line of independently driven presses, each equipped with a variable speed drive, a digital control system in which respective digital signals representative of the instantaneous phase positions of 211 Appl. No.: 45,316

[52] US. Cl ..318/85 the several presses in the line are each differentially compared [51] Int. Cl. ..H02p 5/50 against a digital reference signal representative of reference Field of Search phase position. As a result of the differential comparisons, if

307/252 w, 233 the phase of any given press leads or lags the reference phase, a phase error is detected. This phase error is converted to an [56] References Cited analog control signal which has a sense dependent on whether UNITED STATES PATENTS the press phase is leading or lagging the reference phase and a magnitude dependent on the amount of such lead or lag, and

2,928,033 3/1960 Ahbott ..3l8/604 the ontrol signal is fed back to the press drive to speed up or 3,408,549 10/1968 shlmabllkufo-m slow down the press as necessary to bring it back into step 3,064,173 11/1962 Bree" "3318/85 The digital reference phase position signal cycles at the 3,333,089 7/1967 Saylor ..3l8/632 desired line operating speed so that a presses operate in synchronism at that speed.

5 Claims, 4 Drawing Figures DIGITAL CONTROL OF PRESS SYNCIIRONIZATION BACKGROUND OF THE INVENTION The present invention relates to power presses, and more particularly to a digital control system for synchronizing the operation of a line of independently driven presses.

As will be appreciated, press line synchronization presents a fairly unique control problem, since it requires that several presses, each generally having a tremendous inertia, be kept in step with one another, despite that the load on any given press varies with time and that at any given time the loads on the respective presses may differ considerably. Indeed, until relatively recently, there was no control system for automatically synchronizing a line of independently driven power presses. Instead, the benefits of having the presses independently driven or operable in a continuous mode were sacrificed in the interest of synchronization. For example, one approach that was taken was to interconnect the drive shafts of the several presses to thereby mechanically and forcibly hold the presses in step. But, this was not a satisfactory solution, since the presses generally all had to have substantially the same crown heights to facilitate the mechanical interconnection of their drive shafts. Also, it was very difficult and time consuming to add or remove presses from the line, and the common drive shaft was subjected to extreme torques. Another approach that was taken was to use the operation of one press to trigger the operation of the next, and so on down the line. But, again, this was not a satisfactory solution, primarily due to the severe limitations it placed on the attainable line operating speed. A further approach was to provide means for an operator to manually adjust the relative phase positions of the several presses, together with a limit switch to shut down the line when the phase error between the fastest and slowest press therein exceeded a predetermined limit, so that the line could be manually readjusted whenever it got out of synchronism. This approach not only required the frequent intervention of an operator, but also substantially prevented operation of the line in a continuous mode, since the limits of tolerable phase error were generally exceeded in no more than approximately twenty strokes.

However, the basic problem was finally solved, as disclosed and claimed in Danly U.S. Pat. No. 3,199,439, by recognizing that the operation of a line of independently driven presses,

each equipped with a variable speed drive, could by synchronized at any desired operating speed, up to a limit imposed by the mechanical capabilities of the presses, by feeding back to each press drive a phase error signal obtained by continuously comparing the press phase position against an externally generated reference phase position. With that arrangement, if any given press tends to get out of step, a phase error signal is supplied to the press drive to increase or decrease the press speed as necessary to maintain the press in step. Thus, any corrective action required is provided when only minor changes in press speed suffice and, therefore, the necessary corrections are readily made despite the high inertias of the presses.

SUMMARY OF THE INVENTION An object of the present invention is to provide a relatively compact and lightweight control system for synchronizing the operation of a line of independently driven presses. A related object is to provide a control system of the foregoing type which requires only negligible amounts of power for its operation. More specifically, it is an object of this invention to provide a digital control system, which permits any number of standard or custom built presses to be operated as a closely synchronized press line.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the present invention will become apparent when the following detailed description is read in connection with the attached drawings, in which:

FIG. 1 is a side elevation, partly diagrammatic, to illustrate a portion of a typical press line with which the present invention can be advantageously employed;

FIG. 2 is a block diagram of an exemplary embodiment of the control system of the present invention, and illustrates its association with an exemplary one of the presses shown in FIG. 1; and

FIGS. 3A and 33, when interconnected as indicated at X- X, form a schematic diagram of a combined add-subtract digital counter and digital to analog converter suitable for use in the control system illustrated in FIG. 2.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT While the invention will be described in connection with a single illustrated embodiment, it is to be understood that the intent is not to limit it to that embodiment To the contrary, the intent is to cover all alternatives, modifications and equivalents that come within the spirit and scope of the invention.

THE TYPICAL PRESS LINE Turning now to the drawings, and particularly to FIG. 1, there is shown a portion of an automated press line for performing successive operations on a workpiece 11 as the workpiece passes down the line. Only three presses l2, l3 and 14 are shown, but it will be understood that in practice the present invention may be used to synchronize the operation of any number of presses.

As here illustrated, the presses are identical and of generally conventional construction. For example, the press 12 includes a massive base 15, which is anchored below the floor 16, and a frame 17 topped by a crown 18. Reciprocatingly mounted in the press frame 17 there is a slide, diagrammatically indicated at 19, with cooperating upper and lower dies 21 and 22, respectively. To drive the press 12, a motor 23 is mounted on the press crown 18 and coupled to the press drive or crank shaft 24 (FIG. 2) which, in turn, is coupled to the press slide 19.

The coupling between the press motor 23, crank shaft 24 and slide 19 is not shown in detail, since it may be entirely conventional. If the details of the coupling are of interest, reference may be had to the trade brochures and descriptive bulletins of the press manufacturers, as well as to Danly U.S. Pat. Nos. 3,199,439 and 3,199,443. For present purposes it suffices to simply note that the press drive motor 23 and its associated speed control 25 provide an electrically responsive, variable speed drive for the press 12, and that the drive for the press 12 is independent of the similar variable speed drives provided for the other presses l3 and 14 in the line.

As illustrated, for transporting the workpieces 11 down the press line, there are respective conveyors 26 leading up to and away from the input and the output sides of each press. Further, each press is equipped with an input transfer mechanism 27 and an output transfer mechanism 28. The input and output transfer mechanisms for any given press, as well as at least one of the adjacent conveyors 26, are coupled (by means not shown) to the press drive motor 23 for operation in coordination with the movement of the press slide 19. The details of the transfer mechanisms 27 and 28 and of the coupling between the press drive motor 23, transfer mechanisms 27 and 28, and associated conveyors 26 form no part of the present invention. Therefore, for a discussion of such details, reference is again made to the aforementioned Danly US. Pat. Nos. 3,199,439 and 3,199,443. For now it suffices to note that the coordination of the conveyors 26 and transfer mechanisms 27 and 28 with their associated presses provides a generally continuous flow of workpieces 11 along the line, while assuring that each press is loaded and unloaded during its operating cycle at predetermined points selected to assure ample clearance between the dies 21 and 22 for free movement of one workpiece out of and another workpiece into the work area of the press.

THE CONTROL SYSTEM With the characteristics of a typical press line now in mind, attention is directed to the control system provided in accordance with the present invention for automatically synchronizing the operation of the press line. Referring to FIG. 2, the control system is described with reference to the control of the phase position of only the press 12, since the control circuits for the other presses are identical to the one press control circuit 31 shown. It will, of course, be understood that the description of the control system as applied to the press 12 is equally applicable to the control system as applied to any of the other presses.

In accordance with the present invention, a digital signal representative of the phase position of the press 12 is differentially compared against a digital signal representative of a reference phase position, so that if there is any lead or lag in the press phase position relative to the reference phase position, a phase error signal is obtained. The phase error signal is presented as an analog signal, which has a sense dependent on whether the phase of the exemplary press 12 is leading or lagging the reference phase and a magnitude dependent on the amount of any such lead or lag. The analog signal is, in turn, supplied as a feedback signal for correctively energizing the press drive to slow down or speed up the press as necessary to eliminate the lead or lag.

More particularly, the press control circuit 31 includes a pulse generator 32, which is driven at the operating speed of the press 12, such as by being coupled directly to the press crank shaft 24, to supply a digital signal which is representative of the existing press phase position. The digital press phase position signal uniquely identifies the existing press phase position and changes with each incremental change of such position, say each 1, through the full 360 of the press cycle. As shown, the press phase position signal is applied to one input of an add-subtract digital counter 33 and also to a press stroke position indicator 34. The position indicator 34 is a conventional digital counter, which supplies a readout from which an operator can determine the press phase position at any given instant. Typically, it is automatically reset (by means not shown) when the press phase passes through its 360 phase cross over point.

An external pulse generating means 35 supplies a digital phase position reference signal which uniquely identifies the existing reference phase position and which cycles at a frequency or repetition rate corresponding to the desired operating speed for the press line. The phase position reference signal is applied to the second input of the add-subtract digital counter 33 and to a stroke position indicator 36. The stroke position indicator 36 again comprises a conventional counter, which is automatically reset (by means not shown) when the reference phase position signal passes through its 360 phase crossover point. It provides a readout from which the operator can determine the reference phase position at any given instant.

Of course, the digital reference phase position signal may be supplied in a number of different ways. For example, as shown, the usual control console for the control line may include a reference pulse generator 37 driven by a master motor 38. Unlike the press motor 23, the load on the master motor 38 remains substantially constant. Thus, once the speed of the master motor 38 is adjusted to the desired operating speed for the press line, it remains substantially constant.

To eliminate any lead or lag of the phase of the press 12 relative to the reference phase, the digital signals supplied by the pulse generators 32 and 37 are differentially compared in the add-subtract digital counter 33 thereby providing a phase error signal, which is converted in a digital to analog converter 39 to supply an analog control signal that has a sense dependent on whether the press phase leads or lags the reference phase and a magnitude dependent on the amount of the lead or lag. The control signal is, in turn, fed back to the press drive speed control means, herein referred to as a press drive motor control 25, to correctively energize the press drive for speeding up or slowing down the press 12 as necessary to bring the press phase back into step with the reference phase. As will be appreciated, similar corrections are provided when necessary for each of the other press operating speeds, so that each press in the line is in phase with the reference phase and, therefore, in step or synchronism with each of the other presses in the line.

While the details of the pulse generators 32 and 37 are not here shown, it will be understood that they may suitably be conventional units, such as binary coded decimal discs. As far as the present invention is concerned, the. one important characteristic of the pulse generators 32 and 37 is that they supply respective digital signals which can be procesed to yield an analog control signal with a sense dependent on whether the phase of the exemplary press 12 leads or lags the reference phase and a magnitude dependent on the amount of any such lead or lag. It will, of course, be understood that by proper selection of the add-subtract digital counter 33 and the digital to analog converter 39, a variety of digital signals could satisfy the above requirements. However, for purposes of the further description of the illustrated embodiment, it will be assumed that the pulse generators 32 and 37 are binary coded decimal discs and that the digital signals supplied thereby are both based on the same predetermined binary coded decimal system.

Further, it will be understood that the synchronized presses are not necessarily in phase with one another in the sense that their slides are necessarily at the same stroke positions at any given instant. To the contrary, the zero phase position for each press is selected so that as the press operates in phase with the reference, its stroke is timed relative to the strokes of the other presses for the most efficient performance of the operation of the particular press on the workpiece 11. Thus, one press may be at top stroke, while another press is at bottom stroke or any other stroke position.

A SUITABLE COUNTER-CONVERTER Turning now to FIGS. 3a and 3b, to provide an even fuller understanding of the present invention, there is shown an exemplary circuit for performing the functions of the add-subtract digital counter 33 and the digital to analog converter 39 in the digital press synchronizing system of FIG. 2. Specifically, FIGS. 3a and 3b, when joined as indicated at X-X, illustrate a combined counter-converter which differentially compares the digital press and reference phase position signals and converts whatever difference there may be therebetween from digital to analog form, thereby providing for the press drive motor control 25 a control signal which has a sense dependent on whether the press phase is leading or lagging the reference phase and a magnitude dependent on the amount of any such lead or lag so as to speed up or slow down the press 12 as necessary to bring it back into step or synchronism with the other presses in the line. As will be appreciated, the counter-converter of FIGS. 3a and 3b carries out the functions of both the add-subtract digital counter 33 and the digital to analog converter 39 of FIG. 2.

The power requirements of the counter-converter are quite nominal and are primarily dictated by the power required by r the press speed control means; i.e., the press drive motor control 25. Such power as is required can normally be supplied by a readily available line source, such as the volt A.C. source indicated. Thus, there is shown an input transformer 43, which has its primary winding 44 connected across the source and its secondary winding 45 feeding a full wave rectifier 46. To stabilize the voltage developed across the secondary winding 46, the input transformer 43 may further include a tertiary or stabilizing winding 47 tuned in the usual manner to the line frequency by a parallel connected capacitor 48. The full wave rectifier 46 here illustrated comprises a pair of transformers 49 and 50, which have their respective primary windings 51 and 52 connected in parallel across the input transformer secondary winding 45 and their respective split secondary windings 53, 54 and 55, 56 connected in parallel through respective diodes 57, 58, 59 and 60 between a pair of buses W and 'Z. The transformers 49 and 58 are oppositely wound (as indicated by the dot notation) and the diodes 57-60 are poled for conventional full wave rectification, so that the buses W and Z are supplied with DC potentials, which have ripple frequencies twice as high as the source frequency and which are positive and negative, respectively, relative to a common bus U.

As previously mentioned, in the illustrated embodiment, the digital signals supplied by the pulse generators 32 and 37 are based on the same predetennined binary coded decimal system. For example, the digital press phase position signal supplied by the pulse generator 32 comprises a plurality of binary digits or bits, each of which has two possible states 1" or 0, such as a high voltage level and a low voltage level, to indicate the presence or absence, respectively, of a predetermined amount of angular rotation in the particular press phase position existing at any particular time. Accordingly, to derive the desired control signal from the digital press and reference phase position signals, the counter-converter comprises identical but oppositely poled summing banks 41 and 42 for converting the digital press and reference phase position signals to respective corresponding analog signals. The summing banks 41 and 42 are symmetrically connected via the common bus U to the press drive motor control 25, so that their respective analog signals are applied thereto in bucking relationship, with the result that the net energization or control signal for the press drive motor control 25 has a sense dependent on whether the press phase leads or lags the reference phase and a magnitude dependent on the amount of any such lead or lag.

As is apparent, with the exception of the aforementioned polarity difference, the summing banks 41 and 42 are identical. Taking the summing bank 41 as being exemplary, it will be seen that to convert the digital press phase position signal supplied by the pulse generator 32 into a corresponding analog signal, the summing bank includes for each bit of the digital press phase position signal a corresponding switching device 61-70 and series connected weighting resistors 72-81. As here depicted, the switching devices 61-70 are silicon controlled rectifiers, which have their anode-cathode power circuits connected in parallel between the positive D.C.' bus W and the common bus U, with the result that the analog signal supplied by the summing bank 41 is the summation of any and all current flow through the anode-cathode circuits of the controlled rectifiers 61-70. The bits of the digital press phase position signal are separately applied as indicated to the gates of the corresponding controlled rectifiers 61-70, so that each of the controlled rectifiers operates in a conductive or nonconductive state in dependence on the logic level of the particular bit applied thereto; i.e., in a conductive state when the bit is at a high voltage (or 1") level and a non-conductive state when the bit is at a low voltage (or 0) level. The weighting resistors 72-81 are connected in series in the anodecathode circuits of the controlled rectifiers 61-70, respectively, and their values are selected so that the current drawn by the controlled rectifiers 61-70 in response to any given digital press phase position signal is proportional to the press phase position represented thereby. In keeping with accepted practices, each of the controlled rectifiers 61-70 is further shown as having a respective resistor 94-103 connected across its gate-cathode circuit to provide a path for reset current flow.

Of course, the press and reference phase positions continuously and repetitively cycle through 360 and, therefore, may at times be on opposite sides of their respective phase crossover points; say at and 350. As will be appreciated, under such circumstances, the appropriate control signal for the press drive motor control cannot be derived simply by differentially comparing or bucking analog signals representative of the respective phase positions against one another. To the contrary, it is necessary to increase the level of the analog signal for the phase position that is past its phase crossover point by an amount proportional to 360 of angular rotation to provide a proper basis for comparing it with the analog signal representative of the other phase position. To this end, the summing banks 41 and 42 of the counter-converter shown further include respective auxiliary circuits and 106. When the press phase position is leading and past its phase crossover point while the reference phase position is still short of its phase crossover point, the auxiliary circuit 105 increases the total current supplied by the summing bank 41 by an amount proportional to 360 of angular rotation. On the other hand, when the press phase position is lagging and still short of its phase crossover point while the reference phase position is past its phase crossover point, the auxiliary circuit 106 increases the total current supplied by the summing bank 42 by an amount proportional to 360 of angular rotation.

Again, with the exception of the polarity difference, the auxiliary circuits 105 and 106 are essentially identical. Therefore, referring to the auxiliary circuit 105 for descriptive purposes, it will be seen that it includes a silicon controlled rectifier 111, which has its anode-cathode circuit connected through a weighting resistor 112 between the positive D.C. bus W and the corrunon bus U. Means are coupled to the gate of the controlled rectifier 111 to fire it when the press phase position is leading and past its phase crossover point while the reference phase position is still short of its phase crossover point. The value of the weighting resistor 112 is selected so that when the controlled rectifier 111 is fired, the total current supplied by the summing bank 41 is increased by an amount proportional to 360 of angular rotation.

To permit the use of a simple and reliable means for firing the controlled rectifier 111, the number of bits of the digital press and reference phase position signals relied on in determining whether the additional current flow available from the auxiliary circuit 105 is required or not is preferably minimized. Indeed, in the interest of minimizing the number of bits involved, it is permissible to sacrifice the ability to discriminate between leading and lagging press phase conditions in those situations in which the absolute error between the press and reference phase positions exceeds some relatively large angle, such as 100. Of course, such a large error is unlikely to occur during normal operation and, if it does occur, the inability to accurately determine whether the press phase position is leading or lagging will only slightly affect the time consumed in bringing the press back into synchronism. For example, as here depicted, the controlled rectifier 111 is fired whenever the press and reference phase positions are within ranges of O-200 and 300-360, respectively, since the determination as to whether they are within such ranges or not requires that the levels of only three of the twenty bits of the digital press and reference phase position signals be monitored; i.e., the 200 bit of the press phase position signal and the 100 and 200 bits of the reference phase position signal. In particular, as here illustrated, the impedance between the gate-cathode circuit of the controlled rectifier 111 is regulated in dependence on the press phase position and the current flow through such impedance is regulated in dependence on the reference phase position, such that the combination of high impedance and high current required to develop a firing pulse for the controlled rectifier 111 exists only when the press and reference phase positions are within the aforementioned ranges of 0-200 and 300-360, respectively.

More specifically, for regulating the impedance in the gatecathode circuit of the controlled rectifier 111 in dependence on the press phase position, a gate-cathode resistor 113 is shunted by the collector-emitter circuit of a transistor 114. The transistor 114 has its base connected via a current limiting resistor 115 and a blocking diode 116 to receive from the digital press phase position signal the bit representative of a press phase position of 200. Thus, it will be seen that when the press phase position is between 0 and 200 the transistor 114 is held in a non-conductive state and the impedance appearing across the gate-cathode circuit of the controlled rectifier 111 is relatively high, whereas when the press phase position is between 200 and 360 the transistor 114 is held in a conductive state and the impedance appearing across the gate-cathode circuit of the controlled rectifier 111 is then comparatively low.

n the other hand, for regulating the current flow through the impedance in the gate-cathode circuit of the controlled rectifier 111, connected in series with the parallel combination of the resistor 113 and collector-emitter circuit of the transistor 114, between the positive D.C. bus W and the common bus U, there is a pair of photoconductive diodes 117 and 118 and a current limiting resistor 119. Further, aligned with each of the photoconductive diodes 117 and 118 there is a respective light source or lamp 121 and 122. The energization circuits for the lamps 121 and 122 are completed, as indicated by the terminals F and G, in the summing bank 42 via silicon controlled rectifiers 123 and 124, respectively, which are switched into and out of conduction in dependence on the logic levels of the 100 and 200 bits, respectively, of the digital reference phase position signal. The photoconductive diodes 117 and l 18 are selected so that sufficient current fiow is available to develop a firing pulse for the controlled rectifier 111 only when both of the photoconductive diodes are illuminated by their respective lamps 121 and 122 and, thus, only when the reference phase position is between 300 and 360.

From the foregoing it will be understood that the present invention provides a relatively simple and compact digital control system for synchronizing the operation of a line of independently driven presses for operation thereof in a continuous mode, and that the control system does not impose any direct limitations on the sizes or number of the presses that may be included within the line.

I claim as my invention:

1. A control system for automatically synchronizing a plurality of independently and cyclically driven presses, each having an electrically responsive, variable speed drive; said system comprising a cyclically rotating reference;

first means coupled to said reference for supplying a first digital signal representative of the instantaneous phase position of said reference; and, for each of said presses,

second means coupled to the press drive for providing a second digital signal representative of the instantaneous press phase position;

third means coupled to said first and second means and responsive to said first and second signals for differentially comparing the press phase position against the reference phase position and for converting any phase difference therebetween to an analog error signal having an amplitude and polarity respectively indicative of the amount and sense of said phase difference;

fourth means coupled to said first and second means and activated in response to said first and second signals only when the press and reference phase positions are in predetermined bands at opposite sides of but adjacent to their respective phase crossover points for supplying another analog signal having an amplitude and polarity representative of 360 of rotation of whichever one of said phase positions is leading;

means coupled to said third and fourth means for summing said analog error signal and said other analog signal to supply an analog control signal normally having an amplitude and polarity respectively representative of the amount and sense of a minimum difference between the press and reference phase positions; and

means for applying said control signal to correctively energize the press drive.

2. The control system of claim 1 wherein said reference is external of said presses whereby said first digital signal cycles at a rate independent of any variations in the operating speeds of said presses, and wherein said presses comprise a press line for performing successive operations on a workpiece.

3. The control system of .claim 7 wherein said first and second digital signals are based on a binary coded decimal system such that each of them comprises a plurality of bits each of which has a first and second logic level to indicate the presence and absence, respectively, of a predetermined amount of angular rotation; said third means comprises a counter-converter having first and second pluralities of oppositely poled switching devices coupled to the press drive; each of said first switching devices having an input connected to receive a respective one of the bits of said first digital signal and an output connected through a respective weighting resistor in parallel with the outputs of the other of said first switching devices whereby said first switching devices are switched into and out of conduction in dependence on the logic levels of the bits of said first digital signal to supply a first analog signal which is of predetermined polarity and representative of the reference phase position; and each of said second switching devices having an input connected to receive a respective one of the bits of said second digital signal and an output connected through a respective weighting resistor in parallel with the outputs of the other of said second switching devices whereby said second switching devices are switched into and out of conduction in dependence on the logic levels of the bits of said second digital signal to supply a second analog signal which is of opposite polarity and representative of the phase position of said press.

4. The control system of claim 3 wherein said fourth means includes first and second auxiliary circuit means connected in parallel with said first and second switching devices, respectively; said first auxiliary circuit means being responsive to the logic levels of predetermined bits of said first and second digital signals for increasing said first analog signal by an amount proportional to 360 of rotation when the press phase position is lagging and within a predetermined range of positions short of its phase crossover point while the reference phase position is within a predetermined range of positions past its phase crossover point; and said second auxiliary circuit means being responsive to the logic levels of predetermined bits of said first and second digital signals for increasing said second analog signal by an amount proportional to 360 of rotation when the press phase position is leading and within a predetermined band of positions past its phase crossover point while the reference phase position is within a predetermined band of positions short of its phase crossover point.

5. The control system of claim 4 wherein said reference is external of said presses whereby said first digital signal cycles at a rate independent of any variations in the operating speeds of said presses, and wherein said presses comprise a press line for performing successive operations on a workpiece. 

1. A control system for automatically synchronizing a plurality of independently and cyclically driven presses, each having an electrically responsive, variable speed drive; said system comprising a cyclically rotating reference; first means coupled to said reference for supplying a first digital signal representative of the instantaneous phase position of said reference; and, for each of said presses, second means coupled to the press drive for providing a second digital signal representative of the instantaneous press phase position; third means coupled to said first and second means and responsive to said first and second signals for differentially comparing the press phase position against the reference phase position and for converting any phase difference therebetween to an analog error signal having an amplitude and polarity respectively indicative of the amount and sense of said phase difference; fourth means coupled to said first and second means and activated in response to said first and second signals only when the press and reference phase positions are in predetermined bands at opposite sides of but adjacent to their respective phase crossover points for supplying another analog signal having an amplitude and polarity representative of 360* of rotation of whichever one of said phase positions is leading; means coupled to said third and fourth means for summing said analog error signal and said other analog signal to supply an analog control signal normally having an amplitude and polarity respectively representative of the amount and sense of a minimum difference between the press and reference phase positions; and means for applying said control signal to correctively energize the press drive.
 2. The control system of claim 1 wherein said reference is external of said presses whereby said first digital signal cycles at a rate independent of any variations in the operating speeds of said presses, and wherein said presses comprise a press line for performing successive operations on a workpiece.
 3. The control system of claim 7 wherein said first and second digital signals are based on a binary coded decimal system such that each of them comprises a plurality of bits each of which has a first and second logic level to indicate the presence and absence, respectively, of a predetermined amount of angular rotation; said third means comprises a counter-converter having first and second pluralities of oppositely poled switching devices coupled to the press drive; each of said first switching devices having an input connected to receive a respective one of the bits of said first digital signal and an output connected through a respective weighting resistor in parallel with the outputs of the other of said first switching devices whereby said first switching devices are switched into and out of conduction in dependence on the logic levels of the bits of said first digital signal tO supply a first analog signal which is of predetermined polarity and representative of the reference phase position; and each of said second switching devices having an input connected to receive a respective one of the bits of said second digital signal and an output connected through a respective weighting resistor in parallel with the outputs of the other of said second switching devices whereby said second switching devices are switched into and out of conduction in dependence on the logic levels of the bits of said second digital signal to supply a second analog signal which is of opposite polarity and representative of the phase position of said press.
 4. The control system of claim 3 wherein said fourth means includes first and second auxiliary circuit means connected in parallel with said first and second switching devices, respectively; said first auxiliary circuit means being responsive to the logic levels of predetermined bits of said first and second digital signals for increasing said first analog signal by an amount proportional to 360* of rotation when the press phase position is lagging and within a predetermined range of positions short of its phase crossover point while the reference phase position is within a predetermined range of positions past its phase crossover point; and said second auxiliary circuit means being responsive to the logic levels of predetermined bits of said first and second digital signals for increasing said second analog signal by an amount proportional to 360* of rotation when the press phase position is leading and within a predetermined band of positions past its phase crossover point while the reference phase position is within a predetermined band of positions short of its phase crossover point.
 5. The control system of claim 4 wherein said reference is external of said presses whereby said first digital signal cycles at a rate independent of any variations in the operating speeds of said presses, and wherein said presses comprise a press line for performing successive operations on a workpiece. 